A direct-coupled-resonator filter has been conventionally known as a band-pass filter for microwaves. A direct-coupled-resonator filter is configured to include a plurality of resonators which are coupled in series (for example, N resonators, where N is an integer greater than or equal to 2). A first resonator is connected to a signal source, and a last resonator is connected to a load resistor. A coupling degree between the first resonator and the signal source, and a coupling degree between the last resonator and the load resistor are each referred to as an external Q factor or referred to as an external coupling coefficient. The present disclosure utilizes the term “external Q factor Qe.”
Non-patent Literature 1 discloses the following Expression (1), which defines a relationship between (i) a frequency band of electromagnetic waves passed by a filter and (ii) the external Q factor Qe, which relationship occurs in a case where a plurality of resonators are symmetrically configured (i.e., a configuration of the plurality of resonators when viewed from a side of the first resonator is equivalent to a configuration of the plurality of resonators when viewed from a side of the last resonator).Δω3dB=Δω+−Δω−=ω0/(Qe/2)  [Expression (1)]
Here, ω0 denotes a center frequency at which |S21|, an S-parameter expressing transmission, becomes greatest; Δω3dB denotes a bandwidth of a band (hereinafter, referred to as a “3 dB band”) where |S21| is attenuated by 3 dB in comparison to |S21(ω0)|; Δω− denotes a lower limit frequency of the 3 dB band; and Δω+ denotes an upper limit frequency of the 3 dB band.
Expression (1) can be expressed as in Expression (2), and Expression (2) can be expressed as in Expression (3).Qe=2ω0/(Δω+−Δω−)  [Expression 2]Qe=2f0/(f2−f1)  [Expression 3]
In Expression (3), f0 denotes a center frequency corresponding to the abovementioned center frequency ω0; f1 denotes a lower limit frequency, of the 3 dB band, corresponding to Δω−; and f2 denotes an upper limit frequency of, the 3 dB band, corresponding to Δω+.
It is known that the external Q factor Qe is proportional to the inverse of a fractional bandwidth of a filter. It is therefore possible to realize a filter having a desired fractional bandwidth by adjusting the external Q factor Qe of the filter to a desired value.
FIG. 1 and FIG. 10 of Non-patent Literature 2 each illustrate a filter configured by four (4) resonators R1 through R4. The resonators R1 through R4 employ a configuration using a post-wall waveguide technique. A post-wall waveguide is configured to include (i) a dielectric substrate, (ii) a post wall composed of a plurality of conductor posts provided in the dielectric substrate in a fence-like manner, and (iii) an upper and lower wide wall, which are provided on an upper and lower surface of the dielectric substrate, respectively. Note that a post-wall waveguide is also referred to as a substrate integrated waveguide (SIW).
As illustrated in FIG. 10 of Non-patent Literature 2, a part of the resonator R1 which electromagnetic waves enter (or a part of the resonator R4 which electromagnetic waves exit) includes (i) a strip-shaped conductor which is obtained by stretching, in one direction, an electric conductor plate of which an upper wide wall of the resonator R1 (or the resonator R4) is composed, and (ii) slots for separating the strip-shaped conductor from the upper wide wall. The strip-shaped conductor and the lower wide wall constitute a micro strip line. Note that no post wall is provided under the strip-shaped conductor.
With the configuration, the filter of Non-patent Literature 2 makes it possible to alter the external Q factor Qe by altering the length of the slots (see FIG. 9 of Non-patent Literature 2). Note that the results indicated in FIG. 9 were obtained not by using the construction illustrated in FIG. 10, but by using the construction illustrated in FIG. 8.
FIG. 1 through FIG. 3 of Patent Literature 1 illustrate an input/output structure 10, of dielectric waveguide tube, which is included in a dielectric waveguide tube duplexer 11. The input/output structure 10 of the dielectric waveguide tube is composed of a low-frequency-side dielectric waveguide tube resonator 20a, a high-frequency-side dielectric waveguide tube resonator 20b, and a coaxial connector 70. The dielectric waveguide tube resonators 20a and 20b are configured such that a conductor film covers the exterior of a substantially rectangular-parallelepiped-shaped dielectric.
A side surface 20a1 of the dielectric waveguide tube resonator 20a has a coupling window 40a via which the dielectric is exposed. Similarly, a side surface 20b1 of the dielectric waveguide tube resonator 20b has a coupling window 40b via which the dielectric is exposed. The side surfaces 20a1 and 20b1 of the input/output structure 10 are provided opposite to each other so that the coupling windows 40a and 40b are aligned with each other.
Each of the coupling windows 40a and 40b includes a linear-shaped probe 50 made from a conductor film. A first end of the probe 50 is connected to a feed point 60, while a second end of the probe 50 is connected to a conductor film provided in a periphery of the coupling windows 40a and 40b. The coaxial connector 70 is provided on the feed point 60.
As illustrated in FIG. 5 of Patent Literature 1, the dielectric waveguide tube resonator 20a is provided so as to be followed, in series, by dielectric resonators 21a, 22a, and 23a. The dielectric resonators 20a, 21a, 22a, and 23a constitute a resonator group 11a for reception on a low frequency side. Similarly, the dielectric waveguide tube resonator 20b is provided so as to be followed, in series, by dielectric resonators 21b, 22b, and 23b. The dielectric resonators 20b, 21b, 22b, and 23b constitute a resonator group 11b for reception on a high frequency side. The resonator groups 11a and 11b function as a filter.
Patent Literature 1 discloses a technique for adjusting an external Q factor (referred to as “external Q” in Patent Literature 1) by adjusting distances D40a and D40b illustrated in FIG. 3 of Patent Literature 1. The distance D40a refers to a distance between (i) a side surface 20a2, which is contiguous with and orthogonal to the side surface 20a1 of the dielectric waveguide tube resonator 20a, and (ii) a center of the probe 50. Similarly, the distance D40b refers to a distance between (i) a side surface 20b2, which is contiguous with and orthogonal to the side surface 20b1 of the dielectric waveguide tube resonator 20b, and (ii) the center of the probe 50.
Furthermore, Patent Literature 1 discloses, with reference to FIG. 7 of the same, a technique for decreasing the external Q factor by configuring a tip part 51a of a probe 51 to have a width w51a that is greater than a width w51 of the probe 51.